1. Field of the Invention
The invention is directed to techniques for testing and evaluating materials and coatings and, more particularly, to testing and evaluating materials and coatings in moving solutions.
2. Description of Related Art
The techniques of the invention as well as the techniques the prior art are intended to measure the performance of materials exposed to flowing solutions, such as seawater, at velocities designed to subject the material to shear stresses experienced in service. For example the measurement of the performance of antifouling paints under dynamic conditions is necessary to help in assessment of anti-fouling paint film thicknesses required for fouling control between dry dockings of ships, in the selection of materials, in quality assurance, and in understanding performance mechanisms.
Test data derived from such measurements can be used to serve as a guide for predicting the service life of anti-fouling paints in order to calculate the necessary paint thickness to fit specified deployment cycles. The aging of anti-fouling paints in service will vary depending on such factors as: berthing location, geographic area of operation, salinity, pH, and temperature of seawater. One should also note that some areas of a ship are subject to differing hydrodynamic conditions.
Several methods have been developed in the prior art for the dynamic aging of antifouling paints. These include high flow flumes, rotating drums, rotating discs and rotating cylinders. None, however, offer an economic, efficient and well-calibrated system that enables hydrodynamic, physical and chemical conditions to be measured at the material surface. A summary of prior art methods, their characteristics, advantages and disadvantages are presented below.
ASTM D4938 Erosion Testing Of AF Paints Using High Velocity Water (high speed water channel):
The high velocity water tunnel consists of a large pump which forces water through a four-sided rectangular section with diminishing width to generate water flows of up to 18 m/s. For example, the Naval Research Lab Key West facility uses a 950 gpm pump and a 5.5 in high rectangular cross section with widths of 3.28, 1.64, 1.09, 0.82, 0.66, and 0.55 in which generate water velocities of 5, 10, 15, 20, 25, and 30 knots respectively. The advantage of this system is that it is an already established ASTM method and it has been shown to effectively challenge coating systems at a high water velocities. The disadvantages, however, are many. The design requires a large horsepower pump to achieve the high velocity of water flows due to inefficiencies caused by large pressure losses in the system. The system is expensive to build. The system is not suited to test a large number of samples as it can only accommodate one test panel at each velocity. The flow characteristics are poorly defined. The narrow width (0.55 in) required to generate high water velocities preclude the testing of panels with large macrofouling communities.
ASTM D4939 Subjecting Marine AF Coating To Biofouling And Fluid Shear Forces In Natural Seawater (rotating drum):
The rotating drum is the most commonly employed method used to imitate the dynamic flow conditions experienced on a ship hull and they have provided much useful information to the industry. The method requires that a drum with diameter greater than 18 in be rotated in natural seawater at velocities calculated to generate the desired hydrodynamic shear stress. Large diameter drums are capable of testing several panels at a time, however, the panels are usually small and require a curvature to match the radius of the drum which make then unsuitable for subsequent hydrodynamic testing in water, tunnels or boats. The systems require large horsepower motors and are energy inefficient due to losses to the surrounding water. The flow characteristics are also usually poorly defined.
Rotating Disk:
The rotating disk is an established laboratory method that has been used to generate some useful data. The flow characteristics, however, are complex and variable across the test surface and this makes it difficult to use the data to model for full-scale prediction performance.